1. Field of the Invention
The present invention relates to a radiation-curable composition which includes a cyanoacrylate component or a cyanoacrylate-containing formulation, a metallocene component and a polymerizingly effective amount of a photoinitiator to accelerate the rate of cure.
2. Brief Description of Related Technology
Cyanoacrylates generally are quick setting materials which cure to clear, hard glassy resins, useful as sealants, coatings, and particularly adhesives for bonding together a variety of substrates see e.g., H. V. Coover, D. W. Dreifus and J. T. O'Connor, "Cyanoacrylate Adhesives" in Handbook of Adhesives, 27, 463-77, I. Skeist, ed., Van Nostrand Reinhold, New York, 3rd ed. (1990)!.
Ordinarily, upon contact with substrate materials possessing a surface nucleophile, cyanoacrylate-containing compositions spontaneously polymerize to form a cured material. The cured material exhibits excellent adhesive properties to materials such as metals, plastics, elastomers, fabrics, woods, ceramics and the like. Cyanoacrylate-containing compositions are thus seen as a versatile class of single-component, ambient temperature curing adhesives.
As noted, cyanoacrylate polymerization is typically initiated using a nucleophile. The cyanoacrylate anionic polymerization reaction proceeds until all available cyanoacrylate monomer has been consumed and/or terminated by an acidic species.
Although the predominant mechanism by which cyanoacrylate monomers undergo polymerization is an anionic one, free-radical polymerization is also known to occur in this regard under prolonged exposure to heat or light of an appropriate wavelength. See e.g., Coover et al., supra. Ordinarily, however, free-radical stabilizers, such as quinones or hindered phenols, are included in cyanoacrylate-containing adhesive formulations to extend their shelf life. Thus, the extent of any free-radical polymerization of commercial cyanoacrylate-containing compositions is typically minimal and in fact is especially undesirable for at least the reason stated.
With conventional polymerizable compositions other than those containing cyanoacrylate monomers, radiation cure generally presents certain advantages over other known cure methods. Those advantages include reduced cure time, solvent elimination (which thereby reduces environmental pollution, and conserves raw materials and energy) and inducement of low thermal stressing of substrate material. Also, room temperature radiation cure prevents degradation of certain heat sensitive polymers, which may occur during a thermal cure procedure.
Radiation-curable, resin-based compositions are legion for a variety of uses in diverse industries, such as coatings, printing, electronic, medical and general engineering. Commonly, radiation-curable compositions are used for adhesives, and in such use the resin may ordinarily be chosen from epoxy- or acrylate-based resins.
Well-known examples of radiation-curable, epoxy-based resins include cycloaliphatic and bisphenol-A epoxy resins, epoxidized novolacs and glycidyl polyethers. See e.g., U.S. Pat. No. 4,690,957 (Fujiokau) and European Patent Publication EP 278 685.! The common cure mechanism for such radiation-curable epoxy-based compositions is reported to be cationic polymerization.
Well-known examples of radiation-curable, acrylate-based resins include those having structural backbones of urethanes, amides, imides, ethers, hydrocarbons, esters and siloxanes. See e.g., J. G. Woods, "Radiation-Curable Adhesives" in Radiation Curing: Science and Technology, 333-98, 371, S. P. Pappas, ed., Plenum Press, New York (1992).! The common cure mechanism for such radiation-curable, acrylate-based compositions is free-radical polymerization.
European Patent Publication EP 393 407 describes a radiation-curable composition which includes a slow cure cationic polymerizable epoxide, a fast cure free radical polymerizable acrylic component and a photoinitiator. Upon exposure to radiation, the photoinitiator is said to be capable of generating a cationic species which is capable of initiating polymerization of the epoxide and a free radical species which is capable of initiating polymerization of the acrylic component. The polymerizable acrylic component includes monofunctional acrylates and acrylate esters, such as cyano-functionalized acrylates and acrylate esters, examples of which are expressed as 2-cyanoethyl acrylate (CH.sub.2 .dbd.CHCOOCH.sub.2 CH.sub.2 CN) and 3-cyanopropyl acrylate (CH.sub.2 .dbd.CHCOOCH.sub.2 CH.sub.2 CH.sub.2 CN). (See page 5, lines 19-26.) The photoinitiator includes onium salts of Group Va, VIa and VIIa as well as iron-arene complexes, and generally metallocene salts, provided that the material chosen as the photoinitiator is said to be one which is capable of generating both a cationic species and a free radical species upon exposure to radiation. (See page 5, line 56 page 7, line 15.)
Other reported information regarding photopolymerizable compositions includes formulations containing epoxy compounds and metal complexes, such as disclosed in U.S. Pat. No. 5,525,698 (Bottcher).
U.S. Pat. No. 4,707,432 (Gatechair) speaks to a free radical polymerizable composition which includes (a) polymerizable partial esters of epoxy resins and acrylic and/or methacrylic, and partial esters of polyols and acrylic acid and/or methacrylic acid, and (b) a photoinitiator blend of a cyclopentadienyl iron complex and a sensitizer or photoinitiator, such as an acetophenone.
In D. B. Yang and C. Kutal, "Inorganic and Organometallic Photoinitiators" in Radiation Curing: Science and Technology, 21-55, S. P. Pappas, ed., Plenum Press, New York (1992), cyclopentadienyl transition metal complexes are discussed with attention paid to ferrocene and titanocene. In the absence of halogenated media, Yana and Kutal report that ferrocene is photoinert, though in the presence of such media and a vinyllic source free radical initiated polymerization may occur.
And in C. Kutal, P. A. Grutsch and D. B. Yang, "A Novel Strategy for Photoinitiated Anionic Polymerization", Macromolecules, 24, 6872-73 (1991), the authors note that "c!onspicuously absent from the current catalogue of photoinitiators are those that undergo photochemical release of an anionic initiating species." The authors also note that ethyl cyanoacrylate is "unaffected by prolonged (24-h) irradiation with light of wavelength &gt;350 nm" whereas in the presence of NCS.sup.-, cyanoacrylate is observed to solidify immediately, generating heat in the process. Though the NCS.sup.- was not in that case generated as a result of irradiation, it was generated from the Reineckate anion upon ligand field excitation thereof with near-ultraviolet/visible light.
While metallocenes (such as ferrocenes) have been employed in acrylate-based anaerobic adhesive compositions see e.g., U.S. Pat. Nos. 3,855,040 (Malofsky), 4,525,232 (Rooney), 4,533,446 (Conway) and EP '407!, it is not believed that to date a cyanoacrylate-based adhesive composition has been developed including therein a metallocene at all, particularly with respect to curing through a photoinitiated mechanism.
Accordingly, a photocurable composition including a cyanoacrylate component, a metallocene component and a photoinitiator component would be desirable as possessing the benefits and advantages of cyanoacrylate-containing compositions while curing through at least a photo-induced polymerization mechanism.